It's the start of May 2016.
I went into last winter with 4 apparently healthy colonies.
At the start of April I was optimistic that they had all come through the winter. On those few warm decent days there was a lot of activity.
At the start of May, 2 weeks ago, I went down, 2 colonies were either gone or now non existent / dead.
The other 2 seem a bit weak.
I wonder what happened? Could they have swarmed already? I sort of have a hunch that it is too early, the colony wasn't big enough and it has been to cold. Maybe I'm wrong.
I am sure they have not died of disease.
They haven't starved. There is still a lot of food still in the hive.

Conserving wild bees

Research suggests that bumble bee boxes have a very low success rate in actually attracting bees into them. We find that if you create an environment where first of all you can attract mice inside, such as a pile of stones, a drystone wall, paving slabs with intentionally made cavities underneath, this will increase the success rate.

Most bumble bee species need a dry space about the size a football, with a narrow entrance tunnel approximately 2cm in diameter and 20 cm long. Most species nest underground along the base of a linear feature such as a hedge or wall. Sites need to be sheltered and out of direct sunlight.